1. Field of the invention
The present invention relates to a polymer battery pack and a method of making the same, and more particularly, to a polymer battery pack, which is improved in strength against an external impact and can receive a core pack or bare cell of large capacitance.
2. Description of the Prior Art
In general, a polymer battery pack includes a core pack having a pouch type bare cell and a protection circuit member connected to the bare cell, and a plastic case for receiving the core pack. The entire circumference of the plastic case is ultrasonic-welded in a state in which the core pack is received in the plastic case. Here, the pouch type bare cell refers to a lithium polymer battery which includes a pouch formed of cast polypropylene (CPP)/aluminum/nylon (or polyethyleneterephthalate (PET)), and an electrode assembly of positive electrode/separator/negative electrode and polymer electrolyte received in the pouch. For reference, the polymer battery pack or the lithium polymer battery are named so, in order to distinguish them from a battery pack employing liquid electrolyte and a lithium ion battery, respectively.
Meanwhile, such a polymer battery pack has a problem in that as the pouch defining the bare cell and the plastic case enclosing the pouch are poor in strength, the polymer battery pack is easily damaged by an external impact, whereby its reliability is deteriorated. Namely, because aluminum employed in forming the pouch takes the form of a foil and the plastic case enclosing the same is also very thin and poor in strength, they are easily deformed by an external force and easily pierced by a sharp-pointed nail or a needle. If the polymer battery pack is deformed by an external impact or pierced by a sharp-pointed nail or a needle as described above, smoke, flames or an explosion may be produced due to internal short or the like.
In addition, such a polymer battery pack requires a rib margin along the circumference thereof for supersonic-welding the plastic case, wherein such a rib margin occupies at least 0.8 mm (millimeters) width (0.4 mm for each side). Consequently, the size of a core pack (i.e., a bare cell) to be received in the polymer battery pack should be reduced to that extent, which means that the capacitance of battery will be also reduced. That is, because the size of the core pack received in the plastic case is relatively small as compared to that of the plastic case, a core pack of a large capacitance cannot be received in the plastic case. In fact, a conventional polymer battery pack cannot receive a core pack having a capacitance of at least about 680 mAh.